The present invention relates generally to a composite panel material that is suitable for architectural applications. More particularly, it relates to an improved composite panel that includes two skin layers bonded to an expanded or foamed plastic core. The present invention further relates to an advantageous method for making the improved composite panel.
Composite panels made by laminating a solid (non-foamed), thermoplastic core between two thin sheets of metal are well known materials. They are used primarily for architectural and display applications, such as wall coverings and roofing. An example of such composite panels is REYNOBOND(copyright) Aluminum Composite Material, available from Reynolds Metals Company, that is made of two strong sheets of corrosion-resistant aluminum bonded to a solid, low density polyethylene core. Desirable characteristics of composite panels suitable for architectural applications include high stiffness to weight ratio, high bond strength between the metal and the plastic core, and visually flat surfaces. Improvement in any one of these characteristics would result in an improved composite panel provided that the other characteristics are not compromised.
Foamed plastic products are products in which there is a cellular structure. They are generally composed of two phases, a solid phase, i.e., the plastic matrix, and a vapor phase, i.e., the vapor inside the cells. The foam structure can be either open-celled or closed-celled. Open-celled foams have cells that are interconnected, in which the vapor inside can pass freely between the cells. Closed-celled foams have cells that are separated by cell walls and each cell is completely enclosed. Many foamed polymers are known with weight reductions ranging from solid down to 1-2% of the original polymer density.
Methods relating to the expansion of polyethylene resins are disclosed in U.S. Pat. Nos. 3,098,831; 4,473,516; 4,552,708; 4,738,810; and 4,952,352. In general, these references teach methods of expanding a low density polyethylene starting material employing both blowing and cross-linking agents.
U.S. Pat. No. 4,473,516 discloses a method directed to making a foamed thermoplastic article substantially free from surface imperfections by providing a non-foamed molded surface with a foam core.
It is an object of the present invention to provide an improved composite panel that exhibits a higher strength to weight ratio than prior art composite panels.
It is another object of the present invention to provide an improved composite panel that exhibits acceptable bond strength, and a visually flat surface at least equal to those of prior art composite panels.
It is yet another object of the present invention to provide an advantageous method for making the improved composite panel that is economical and results in consistent quality product.
According to one object of the present invention, an improved composite panel comprises a plastic core made from a foamed plastic and at least one skin layer bonded to the plastic core. Examples of suitable foamed plastics include foamed polyethylene, foamed polystyrene, foamed polypropylene, foamed polyvinyl chloride (PVC), foamed polyphenylene-oxide, and copolymers or terpolymers thereof. Within these plastics grades providing higher rigidity foam are preferred. The skin layer is preferably a metallic material. Examples of suitable metallic materials include aluminum alloys, galvanized steel, carbon or stainless steel, copper, titanium, and zinc. Any chemical, or mechanical means, or any combination thereof can be used to bond the skin layer to the plastic core. Preferably, both chemical and mechanical means are employed for enhanced bonding such as, for example, providing an adhesive film layer between the skin layer and the plastic core, and laminating the skin layer with the plastic core at an effective temperature and pressure.
According to another object of the present invention, an improved composite panel comprises a plastic core made from a foamed plastic and two skin layers made from a metallic material. The composite panel exhibits an effective bond strength sufficient to prevent peeling of the skin layers under normal use conditions, at least one visually flat surface substantially free of visual surface defects, and sufficient stiffness to weight ratio preferably at least about 500 lbf-in/lbm/ft2, and more preferably of about 1000 lbf-in/lbm/ft2. Preferably, the composite panel should exhibit a bond strength of at least about 20 in-lb/in, and more preferably at least about 40 in-lb/in.
The present invention further relates to an advantageous method for making the composite panel that includes mixing an effective amount of an encapsulated blowing agent, preferably a chemical blowing agent, with a plastic resin to form a mixture, extruding the mixture at an effective temperature and pressure to form a foamed sheet, and cooling gradually the extruded sheet of expanded plastic resin to form a sheet of foamed plastic having sufficiently flat and smooth surfaces, and a density reduction of from about 10 to about 45 percent, preferably from about 20 to about 45 percent and more preferably from about 30 to about 40 percent, compared to the density of the solid, non-foamed plastic. The method further includes bonding a metallic sheet to the sheet of foamed plastic.